Was Our Oldest Ancestor a Proton-Powered Rock?
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Was our oldest ancestor a proton-powered rock? Nick Lane New Scientist The picture painted by Russell and Martin is striking indeed. The last common ancestor of all life was not a free-living cell at all, but a porous rock riddled with bubbly iron-sulphur membranes that catalyzed primordial biochemical reactions. Powered by hydrogen and proton gradients, this natural flow reactor filled up with organic chemicals, giving rise to proto-life that eventually broke out as the first living cells - not once but twice, giving rise to the bacteria and the archaea. Many details have yet to be filled in, and it may never be possible to prove beyond any doubt that life evolved by this mechanism. The evidence, however, is growing. This scenario matches the known properties of all life on Earth, is energetically plausible - and returns Mitchell's great theory to its rightful place at the very center of biology. This certainly makes abiogenesis sound plausible. Peter Mitchell was an eccentric figure. For much of his career he worked in his own lab in a restored manor house in Cornwall in the UK, his research funded in part by a herd of dairy cows. His ideas about the most basic process of life - how it gets energy - seemed ridiculous to his fellow biologists. "I remember thinking to myself that I would bet anything that [it] didn't work that way," biochemist Leslie Orgel wrote of his meeting with Mitchell half a century ago. "Not since Darwin and Wallace have biology come up with an idea as counter-intuitive as those of, say, Einstein, Heisenberg and Schrödinger." Over the following decades, however, it became clear that Mitchell was right. His vindication was complete when he won a Nobel prize in 1978. Even today, though, most biologists have yet to grasp the full implications of his revolutionary ideas - especially for the origin of life. "Mitchell's ideas were about how cells are organized in space, and cellular energy generation is a feature of that," says geochemist Mike Russell of NASA's Jet Propulsion Laboratory in Pasadena, California. "The problem is that most ideas on the origin of life lack both spatial organization and a supply of energy to drive replication or growth." A few researchers, including Russell, have been rethinking the origin of life in the light of Mitchell's ideas. They think the most counter-intuitive trait of life is one of the best clues to its origin. As a result, they have come up with a radically different picture of what the earliest life was like and where it evolved. It's a picture for which there is growing evidence. Before Mitchell, everyone assumed that cells got their energy using straightforward chemistry. The universal energy currency of life is a molecule called ATP. Split it and energy is released. ATP powers most of the energy-demanding processes in cells, from building proteins to making muscles move. ATP, in turn, was thought to be generated from food by a series of standard chemical reactions. Mitchell thought otherwise. Life, he argued, is powered not by the kind of chemistry that goes on in a test tube but by a kind of electricity. Ancient vents like this one could explain life's strangest features The energy from food, he said, is used to pump positively charged hydrogen ions, or protons, through a membrane. As protons accumulate on one side, an electrochemical gradient builds up across the membrane. Given the chance, the protons will flow back across, releasing energy that can be harnessed to assemble ATP molecules. In energy terms, the process is analogous to filling a raised tank with buckets of water, then using the water to drive a waterwheel. Mitchell dubbed his theory chemiosmosis, and it is not surprising that biologists found it hard to accept. Why would life generate energy in such a complicated and roundabout way, when simple chemical reactions would suffice? It just didn't make sense. It might be counter-intuitive, but chemiosmosis has turned out to be ubiquitous in the living world. Proton power drives not only cell respiration, but photosynthesis too: energy from the sun is converted into a proton gradient in essentially the same way as the energy of food. And proton gradients are often harnessed directly, rather than being used to make ATP. They drive the rotation of the bacterial flagellum, as well as the active transport of numerous substances in and out of cells. So proton power is central to energy generation, movement and maintaining the internal environment - some of the most basic features of life. This suggests that proton power is no late innovation but evolved early in the history of life, an idea supported by the tree of life. The first branch in the tree is between the two great groups of simple cells, bacteria and archaea. Both of these groups have proton pumps and both generate ATP from proton currents, using a similar protein. The obvious explanation is that both inherited this machinery from a common ancestor - the progenitor of all life on Earth. Think about the properties of that common ancestor, however, says Bill Martin of the University of Düsseldorf in Germany, and you come up with a very strange beast indeed. He starts from the assumption that traits found in both the archaea and bacteria are most likely inherited from the common ancestor of all life - though a few have clearly been acquired later by gene exchange. Traits that are distinct presumably evolved independently. There is no doubt that the common ancestor possessed DNA, RNA and proteins, a universal genetic code, ribosomes (the protein-building factories), ATP and a proton-powered enzyme for making ATP. The detailed mechanisms for reading off DNA and converting genes into proteins were also in place. In short, then, the last common ancestor of all life looks pretty much like a modern cell. Yet the differences are startling. In particular, the detailed mechanics of DNA replication would have been quite different. It looks as if DNA replication evolved independently in bacteria and archaea, according to Eugene Koonin at the National Center for Biotechnology Information in Bethesda, Maryland. Beyond that, many biochemical pathways are catalyzed by quite different enzymes. The most surprising and most significant of these is fermentation, the production of energy from food without oxygen. Fermentation is often assumed to be the primordial method of energy generation. Yet Martin has shown that the enzymes responsible are totally unrelated in archaea and bacteria. It looks as if fermentation evolved twice later on, rather than at the dawn of life. Baffling boundaries Even more baffling, says Martin, neither the cell membranes nor the cell walls have any details in common. "At face value, the defining boundaries of cells evolved independently in bacteria and archaea," he says. But if that's the case, what sort of a cell was this common ancestor? A cell with no boundary? Impossible! Something unique? If you exclude the impossible, then whatever you are left with must be true. If Martin is right, the last common ancestor of life on Earth was a sophisticated entity in terms of its genes and proteins, and was powered by proton currents rather than fermentation. Yet at the same time, its bounding membranes were apparently different to anything found today. It was life, but not as we know it. Then, around 2002, Martin came across the work of Russell. Until that time, Russell had been a lone voice. His geochemical ideas about the origin of life didn't go down well with the molecular biologists who dominated the field. From the early 1990s, Russell had been exploring the possibilities of a very particular kind of hydrothermal vent called an alkaline vent, at the time known only from remnants found in ancient rocks. Unlike the black smokers discovered in 1977, formed by the violent reaction of seawater with volcanic lava rising up at the mid-ocean ridges, Russell's vents were much tamer affairs, little more than bubbly rocks riddled with labyrinthine pores. These vents form when water reacts with the mineral olivine, which is common in the sea floor - and would have been even more common early on, before the Earth's crust thickened. The process produces a new mineral, serpentine, and releases hydrogen, alkaline fluids and heat. It also makes the rocks expand and crack, allowing more water to percolate down, sustaining the reaction. The warm, hydrogen-rich effluent ultimately breaks through the sea floor as an alkaline hydrothermal vent. Interest in alkaline vents rose in 2000, when Deborah Kelley and her colleagues from the University of Washington in Seattle stumbled (if one can stumble in a submersible) across an active alkaline vent field just off the mid-Atlantic ridge, exactly where Russell said such vents should be. The team dubbed it the Lost City, partly for its spectacular spires of rock, which form as carbonates precipitate out in the alkaline fluid. Like ancient vents, the spires of the Lost City are riddled with tiny pores, some with dimensions not dissimilar to modern cells. And the chemistry fits the bill too. A report last year confirmed the presence of methane and other small hydrocarbons, as well as hydrogen itself (Science, vol 319, p 604). The vents themselves may be much the same as those around 4 billion years ago, but back then the oceans were very different. The primordial oceans were saturated with carbon dioxide, making them acidic, whereas the seas today are slightly alkaline. And there was practically no oxygen. Without oxygen, iron dissolves readily: the vast banded-iron formations around the world reveal just how much iron was once dissolved in oceans.